1. Field of the Invention
The present invention relates to a method and apparatus for forming a toner layer, which are used in an electrophotographic developing apparatus for developing an image using magnetic toner.
2. Description of the Prior Art
In a conventional method of forming a toner layer used in electrophotographic developing apparatuses, a thin plate member called a Myler is brought into contact with a toner carrier which carries toner on its surface and conveys toner to the developing position. The thickness of the toner layer is regulated by the contact pressure. In another conventional method, a toner layer thickness regulating member called a doctor blade is arranged to oppose a toner carrier, and the thickness of the toner layer is regulated by a gap between the two members.
In the former method, since a toner layer is formed by the contact pressure, wave-like stripes tend to form in the toner layer surface. In order to prevent such stripes, a technique is disclosed in Japanese Patent Disclosure No. 54-137346, and the like. In this technique, the surface of a toner carrier has a three-dimensional pattern and its cross-section has a special profile. However, this technique cannot completely solve the above problem if the toner has poor flowability or toner particles agglomerate.
In the latter method, the thickness of a toner layer is regulated by cutting the toner layer by a doctor blade (to be referred to as ear trim hereinafter). A toner layer formed by this method normally has a thickness which is slightly larger than the gap between the two members. For this reason, a very small gap must be set between the two members. If such a very small gap must be set between the two members with high precision, assembly precision in mounting the doctor blade and machining precision thereof must be high.
In order to solve this problem, a toner layer forming apparatus as shown in FIG. 7 is disclosed in Japanese Patent Laid Open No. SHO 55-103567. In this apparatus, magnetic poles 103 are embedded in the circumferential surface of a fixed magnet assembly 102 enclosed in a toner carrier 101. A magnetic blade 104 is arranged at a position of the toner carrier 101 which opposes the magnetic poles 103. The magnetic blade 104 serves as a toner layer thickness regulating member. Lines of magnetic force Mg act on a gap between the blade 104 and the toner carrier 101.
Referring to FIG. 7, reference numeral 105 denotes a photosensitive drum opposing the toner carrier 101. The drum 105 is closest to the toner carrier 102 at the developing position which is at the downstream side of the mounting position of the blade 104. Reference numeral 106 denotes an AC power source for applying an AC electric field on the toner carrier 101.
According to this basic conventional technique, toner carried on the toner carrier 101 and conveyed toward the developing position is stirred and regulated in layer thickness in the gap between the two members by the lines of magnetic force Mg. Thus, a toner layer having a uniform and stable thickness can be formed at the developing position.
In this basic conventional technique, unlike the other conventional methods, the toner layer thickness is not regulated by the "ear trim" means. Instead, the lines of magnetic force Mg are applied to the toner layer to set a low tone particle density. In this state, the "ear trim" operation by the magnetic blade 104 is performed to regulate the thickness of the toner layer. Thereafter, the toner ears are rearranged or shaped at the outlet side of the gap, thereby forming a reason, the gap can be set to be larger than the desired thickness of the toner layer, specifically, 1.5 times or more. Thus, agglomeration or clogging of toner can be prevented. In addition, if the gap between the two members can be set to be larger than the desired thickness of the toner layer, the mounting precision of the magnetic blade 104 need not be very high. As a result, the problems encountered with the conventional techniques described above can be solved considerably.
In the basic conventional technique, however, since the basic operation involves simultaneous toner stirring and toner layer thickness regulation, the following problem is encountered.
In the basic conventional technique, even if the toner layer density is set low, layer thickness regulation called "ear trim" is still performed by a mechanical slide means i.e., the magnetic blade 104. Therefore, the thickness of the formed toner layer varies in accordance with variations in the gap size.
Although the precision in setting the gap and mounting the magnetic blade 104 is moderated when compared with other conventional apparatuses, a considerably high precision must still be set, resulting in cumbersome adjustment.
In a method of forming a toner layer by simultaneous toner stirring and toner layer thickness regulation, if the toner carrier 101 is rotated at high speed, toner may not be stirred sufficiently before a toner layer is formed. This may result in difficulty in forming a toner layer of uniform thickness.
In accordance with the conventional technique described above, toner tends to deposit on the upstream side wall of the magnetic blade 104. In addition, since the gap between the blade 104 and the carrier 101 is set to be larger than the desired thickness of a toner layer, the deposited toner may enter this gap or a portion downstream of the blade 104. Such entrance of toner may disturb the formed toner layer.
If the thickness of a toner layer is regulated by a mechanical sliding means such as the magnetic blade 104 described above, toner charge is increased by friction between the sliding surface of the blade 104 and the magnetic toner. This will cause variations in the thickness of the toner layer or adversely affect the toner charge characteristics. This also leads to separation of a flowability accelerator held on the toner surface, thereby causing poor flowability of the toner, changes over time in charge characteristics, and agglomeration of toner particles. This adversely affects sharpness of the developed image.